Several drawbacks have been encountered in use when delicate or dangerous loads must be handled with great precision, particularly when it is prohibited to have a human being near the load, to aid in guiding the load in the course of handling.
There is primarily noted a tendency toward twisting of the runs of the traction cable, which twisting gives rise to rotation of the load itself, this rotation being unable to be controlled by the rotatable mounting of the attachment means on its support. Moreover, because the attachment means is free to move along the cable, there is noted a tendency of the load to swing and to slacken along the cable during lifting operations.
The document DE 682 482 discloses a lifting block mounted together with an electric motor drive. The flexible traction means is constituted by metallic strips of small thickness rolled spirally on at least one pair of spaced apart rollers, such that the two strips are simultaneously rolled or unrolled by an identical amount during operations of lifting a load. The use of a strip as enrollable traction means avoids any substantial twisting of the strip, whilst permitting raising the load along a vertical axis.
However, in this known device, the simultaneous rotation in opposite directions of the rollers of at least one pair of rollers requires the use of a high power electric motor coupled to a gear train adapted to drive the two mentioned rollers simultaneously in rotation.
The document EP 0 082 046 discloses an improved hoist with a flexible flat connection constituted by a textile strap or a flat metallic braid. This hoist is generally satisfactory, but has the drawback of being of complicated construction, costly production and difficult maintenance. This hoist is accordingly difficult to use in an environment in which a human cannot be present.